This invention relates to optical interference filters and, more particularly to lamps including an optical interference filter to tailor the transmitted light energy.
Multi-layer optical interference filters and their use with electric lamps are well known to those skilled in the art. Commercially available, high efficiency lamps including an optical interference filter have achieved considerable commercial success such as the MR(trademark) lamps available from General Electric Company. This lamp includes a double ended light source (such as a halogen-incandescent lamp) mounted inside a parabolic reflector. The light source is fabricated from a fused quartz envelope and the parabolic reflector includes a multi-layer coating. In this regard, reflectors have been an essential component of lighting applications for many years. Various types of reflectors are in use ranging from simple polished metal reflectors to coated glass and plastic reflectors. These coated reflectors may have a single layer coating of a reflective metal, a protected reflective metal, an enhanced metal coating or a complex multi-layer coating which can provide both color correction and reduction of unwanted regions of radiation for a specific lighting application. For example, in applications such as movie projectors, slide projectors and overhead viewers it is desirable to reduce the forward transmission of heat or the infrared component of the reflected light as much as possible. These reflectors are known as cold mirrors because of their ability to reduce the amount of heat (infrared radiation) present in the reflected beam.
Typically, cold mirror coatings are based on a high reflectance array consisting of alternating layers of high and low index films, each layer having an optical thickness of one Quarter-Wave Optical Thickness (QWOT). The optical thickness is defined as the product of the physical thickness times the refractive index of the film. The QWOT is referenced to a conveniently chosen design wavelength. For example, at a design wavelength of 1000 nm, a QWOT equals 250 nm. Since a single high reflectance array reflects across only a portion of the visible region, two or more arrays may be combined for an extended high reflectance band across the visible spectrum.
Cold mirror reflector have achieved a high degree of acceptance in display lighting applications where their high degree of reflectance of visible light of the proper color temperature has been found very attractive. Therefore, a combination of high visible reflectance, good color maintenance over the life of the reflector, and the ability to select varying degrees of infrared and ultraviolet reduction have emerged as important factors in lighting coatings.
Optical interference filters are often made of alternating layers of refractory metal oxides having high and low indexes of refraction. Refractory metal oxides are often used because they are able to withstand the relatively high temperatures (e.g 400xc2x0 C. to 900xc2x0 C.) that develop during lamp operation. Such oxides include, for example, titania, hafnia, tantala and niobia for the high index of refraction material and silica or magnesia fluoride for the low index of refraction materials. Examples of these types of filters are provided in U.S. Pat. Nos. 5,143,445 and 5,569,970, wherein these materials provide high reflectance in the visible spectrum between, for example, 380 to 770 nanometers.
Certain uses for these lamps, i.e., photographic and photocopying require low reflected infrared radiation, improved color maintenance and high color temperature. In this regard, the subject invention is provided to minimize the amount of heat which is generated in a forward direction by providing a lamp with an optical interference filter which passes infrared radiation through the back of the reflector.
In an exemplary embodiment of the invention, the lamp of the present invention comprises a light source and a reflector. The reflector includes a coating over a major portion of its inner and/or outer surface. The coating is comprised of multiple layers of high and low index of refraction materials to provide at least 90% reflectance between about 425 and 750 nanometers and less than 20% reflectance peaks between about 900 and 1,800 nanometers, preferably 830 to 1,800 nm.
The present lamp provides a number of advantages over the prior art. For example, the xe2x80x9cright hand edgexe2x80x9d of the reflectance curve has been moved close to 800 nanometers, substantially decreasing the amount of infrared radiation which is reflected. Therefore, a greater amount of infrared radiation passes through the rear of the lamp, reducing the amount of unwanted energy (i.e. energy not providing visible light) transmitted forwardly by the lamp. In addition, the lamp advantageously provides excellent lumen output and a desirable color temperature.